The invention is directed to an electrical component in the form of a chip structure for fastening to a circuit board. The component is composed of a wafer-shaped or lamina-shaped member that is provided with conductive coatings at its large end faces lying opposite one another, and has ribbon-shaped terminal elements for connecting the coatings of opposite polarity to contact locations of a printed circuit situated on the circuit board and that is extrusion-coated with a lamina-shaped or cuboid insulating coat.
Such components are disclosed, for example, by EP-A Ser. No. 229 286 published July 22, 1987.
The invention is also directed to a method for the manufacture of such an electrical component wherein a metal band is first provided with recesses such that conductor webs or ridges are formed, the component members are subsequently inserted between the conductor webs or ridges, connected to the parts of the conductor webs or ridges and provided with an insulating coat.
Such a method is disclosed by EP-A Ser. No. 162 149 published Nov. 27, 1985.
Efforts to accommodate a greater and greater plurality of discrete components on a surface of constant size have led to smaller and smaller housing shapes. The smallest housing shape at present is that of the surface-mounted devices (SMD) that are also referred to as chip components.
Soldering surface-mountable components onto printed circuit boards can be accomplished via various methods. Advantageous methods therefor are immersion soldering, or what is referred to as flow soldering wherein the surfacemounted devices are briefly conducted over a wave of molten soldering tin. This requires the employment of highly heat-resistant materials for the insulation envelope such as, for example, thermoplastics. Until they are secured by soldering, the components are temporarily retained at the circuit board, for example by gluing, with clamps or by pinching into appropriate openings, as disclosed, for example, in DE-AS Ser. No. 1 064 127.
In particular, electrical components within the scope of the present invention are electrical resistors having a value of resistance that is dependent on the applied voltage (varistors), resistors having a value of resistance dependent on the temperature (posistors=PTC resistors or thermistors=NTC resistors), or electrical capacitors having a ceramic or plastic dielectric, whereby all of these components can be executed as cylindrical or cuboid members or as members composed of layers having coatings inserted therebetween. All of these components are disclosed, for example, in the afore-mentioned EP-A Ser. No. 229 286.
In the manufacture of the said components, metal coatings are usually applied to two opposite, large end faces of the member, the terminal elements then being secured to these metal coatings. Subsequently, the component body is provided with an insulating coat preferably of thermoplastic or duroplastic, heat-resistant plastic, for example by compression molding, so that the outer parts of the terminal elements emerge from the insulating coat. The plastic may be potentially filled with insulating coat. The plastic may be potentially filled with mineral or other fillers in a known way.
The present invention can be employed in all components of the type under discussion here, insofar as the components are provided with terminal elements at the opposite, large end faces of the member (surface-contacted), i.e. are not side-contacted.
The afore-mentioned, electrical components have been adequately described in numerous, further publications. The afore-mentioned EP-A Ser. No. 162 149, particularly FIG. 4 and the appertaining description therein, shall be referenced for the specific instance of electrical capacitors in chip structure that are nonetheless provided with power leads at the large, opposite end faces. European Patent EP-A Ser. No. 130 386 published Jan. 9, 1985, also discusses the problem in detail of how the terminal elements of a side-contacted plastic foil wound capacitor are to be conducted through the insulating envelope such and are to be subsequently bent over such at the surface of the envelope that this can be used as a chip component.
FIG. 2 in German Published Application Ser. No. 36 38 286.8, shows SMD varistors that are already commercially available within the framework of the Siemens Series SIOV-CU4032 and are thus Prior Art. FIG. 1 hereof corresponds to the said FIG. 2, and shows that ribbon-shaped terminal elements are provided with bent-off portions between the component body and surface of the insulating coat. Only after this are the terminal elements conducted toward the outside and bent over there to form solder surfaces. Given terminal elements brought directly to the surfaces in parallel (see FIG. 2), parts of unequal length arise at the surface elements of the insulating coat perpendicular to the surface of the circuit board. This leads to an asymmetrical solder moistening during the solder process and causes what is referred to as the tombstone effect that shall be set forth later with reference to the attached FIG. 2.
As may be seen from FIG. 1, the method of bending off the terminal elements within the insulating coat that was previously carried out in fact guarantees an adequate and symmetrical moistening of the outwardly residing contact locations with solder; however, a considerable spatial outlay is required therefor. The insulating requirement in and of itself would definitely not make the hitherto standard, lateral thicknesses of the insulating coat of the components necessary. In normal operation, one can assume about 2,000 V as the highest voltage load of the varistors under discussion here. On the other hand, the plastics that are employed can be loaded with about 40,000 V per mm. Given this insulating coat manufactured in a pressing process, a spatial outlay is exerted that can only be explained on the basis of the specific, spatial arrangement of the terminal elements.